test: fix testlib to not pass an empty argument to 6g

This change is necessary to make the run shell script work
again, but it is not sufficient as bug424.go's execution line
does not name the package that it imports.

R=golang-dev, gri, rsc
CC=golang-dev
https://golang.org/cl/5771043

test/testlib

@@ -28,7 +28,7 @@ run() {
 		esac
 	done
 
-	$G $D/$F.go "$gofiles" && $L $F.$A && ./$A.out "$@"
+	$G $D/$F.go $gofiles && $L $F.$A && ./$A.out "$@"
 }
 
 cmpout() {